In-line optical fiber refractive index gratings are known. See, for instance, U.S. Pat. No. 4,807,950, incorporated herein by reference. See also U.S. Pat. No. 4,725,110, which claims a method of producing such a grating. The currently preferred method of "writing" an in-line grating into optical fiber comprises interference between two beams of actinic (typically UV) radiation in a portion of the fiber. The two beams are incident on the fiber in a transverse direction, the angle between the two beams (and the wavelength of the radiation) defining the grating spacing.
Typically fiber gratings are formed in Ge-doped fiber. See, for instance, F. Ouellette et al., Applied Physics Letters, Vol. 58(17), p. 1813, which inter alia discloses that the sensitivity of the Ge-doped fiber to actinic radiation can be enhanced by a thermal hydrogen treatment (4 hours at 400.degree. C. in 12 atm of H.sub.2). See also G. Meltz et al., SPIE, Volume 1516, International Workshop on Photoinduced Self-Organization in Optical Fiber, May 10-11, 1991, Quebec City, Canada, paper 1516-18, which reports treating a heavily doped germanosilicate preform rod for 75 hours at 610.degree. C. in 1 atm. H.sub.2 to enhance the photosensitivity of the glass. U.S. patent application Ser. No. 643,886, filed Jan. 18, 1991 for R. M. Atkins et al., now U.S. Pat. No. 5,157,747, discloses a process of manufacturing optical fiber that enhances the GeO/GeO.sub.2 ratio in the Ge-doped core of the fiber, thereby enhancing the sensitivity of the fiber to actinic radiation. The process involves, exemplarily, collapsing the preform tube in a substantially oxygen-free atmosphere.
The prior art H.sub.2 sensitization treatments involve exposure of the glass to H.sub.2 at a relatively high temperature, typically at least 400.degree. C. This high temperature treatment would at best be inconvenient if applied to optical fiber. As is well known, optical fiber typically is coated with a polymer material as part of the draw process, since uncoated fiber is fragile and rapidly loses its strength, especially if handled. At the temperatures of the prior art H.sub.2 treatments, typical polymer fiber coatings would be destroyed or at least severely damaged. Furthermore, the prior art high temperature sensitization treatment frequently increases the optical loss in the fiber and/or may weaken the fiber.
D. McStay, SPIE, Vol. 1314, "Fibre Optics '90", pp. 223-233, inter alia reports exposing Ge-doped optical fiber to H.sub.2 for various times at various temperatures and pressures, exemplarily 3 days at 24.degree. C. and 1 atmosphere. Raman measurements were interpreted to reveal the presence of molecular hydrogen in the fiber after the exemplary treatment. Exposure of the fiber to 488 nm radiation resulted in increase of a Raman peak at about 2150 cm.sup.-1. The peak appeared even if irradiation was delayed until after essentially all of the H.sub.2 had again been lost from the fiber. The author disclosed that the observed photosensitive reaction was a weak one, and suggested that a two-photon process may be involved. No refractive index change was observed.
In view of the potential advantages offered by in-line refractive index gratings in optical waveguides, it would be highly desirable to have available a method of locally increasing a waveguide refractive index that is free of the above discussed shortcomings of the prior art. Furthermore, it would be very desirable if strong in-line gratings could be written into optical fiber of the type conventionally manufactured and installed for optical fiber communication systems. This application discloses a method that has these and other advantageous features.